Many RNA viruses utilize internal ribosome entry sites (IRESs) located in the 5′ untranslated region (UTR) of genomic RNA to translate viral proteins in a cap-independent manner. Several cellular host proteins that can bind to and stabilize IRES structures and regulate IRES-driven translation have been reported, and these proteins are known as IRES trans-acting factors (ITAFs). Far upstream element-binding protein 1 (FUBP1, also known as FBP1) and far upstream element-binding protein 2 (KHSRP, also known as KH-type splicing regulatory protein, KSRP, FUBP2, FBP2 or P75) are among the known ITAFs that are functionally important in modulating viral IRES-driven translation.
IRES elements have been shown to exist in all genera of Picornaviridae, a type of cytoplasmic RNA virus known for its infectability on both animals and humans. At present, at least five different types of IRESs have been identified in picornaviruses, and each type is characterized by a distinct secondary structure and a different eIF (eukaryotic initiation factor)/ITAF-requirement. The picornavirus type I IRES is only found in enteroviruses such as enterovirus A71 (EVA71 or EV71), coxsackievirus B3 (CVB3), and human rhinovirus type 2 (HRV2).
EV71 is a positive single-strand RNA virus containing type I IRES in the Picornaviridae family, and is emerging as a potent threat worldwide. EV71 infections normally cause mild diseases, such as hand-foot-and-mouth disease (HFMD) or herpangina. However, children under five years of age are particularly susceptible to the most severe forms of EV71-associated neurological complications, including aseptic meningitis, brainstem and/or cerebellar encephalitis, acute flaccid paralysis (AFP), myocarditis, and rapid fatal pulmonary edema and hemorrhage.
The applicants previously found that FBP2 is a negative regulator (Lin J. Y. et al. (2009), Nucleic Acids Res., 37:47-59), while FBP1 is a positive regulator of EV71 IRES-dependent translation (Huang P. N. et al. (2011), Nucleic Acids Res., 39:9633-9648). Moreover, when the C-terminal of FBP2 is cleaved upon EV71 infection, the cleaved form of FBP2 (FBP21-503) then becomes a positive regulator of EV71 IRES-driven translation. Ubiquitination, and proteasomal and lysosomal activities may be involved in EV71-induced FBP2 truncation (Chen L. L. et al. (2013), J Virol., 87:3828-3838).
By conducting research, the applicants surprisingly found that polypeptides derived from FBP1 and FBP2, respectively, can enhance viral IRES activity and increase viral yield. Therefore, these FBP1 and FBP2 polypeptides are expected to be useful in vaccine production.